cells alive 7: exocytosis and endocytosis Flashcards
what are the functions of the endoplasmic reticulum?
- protein biosynthesis
- lipid biosynthesis
- intracellular calcium store
what is the structure of the endoplasmic reticulum?
- made of tubules and sacs surrounded by membranes
- the tubules and acs are developed from the nuclear outer membrane and connected between each other. they protude into the cytoplasm
- the space encased enclosed by the membranes is called the lumen or ER cysternal space
what is the function of the rough endoplasmic reticulum?
protein synthesis
- ER and golgi residents
- secretory
- transmembrane
- lysosomal
What is the function of the smooth endoplasmic reticulum?
Lipid synthesis - cholesterol and sphingolipids
What is the transitional endoplasmic reticulum?
Areas where vesicles bud from to go to the Golgi apparatus
What post-translational protein modifications can occur in the lumen of the ER?
- Formation of disulfide bonds between cysteines to stabilise the protein
- Proper folding
3 Addition and processing of carbohydrates (N-linked glycosylation-Asn)
- Specific proteolytic cleavages
- Assembly into multimeric proteins
what is N-linked glycosylation-ASn in post translational modifications of the ER
attachment of multiple branched sugars to amide nitrogen of an Asn to stabilise, protect from degradation, hold in the ER and serve as a signal
Describe the structure of the Golgi apparatus
Located near the nucleus and close to the centrosome
Divided into cisternae which communicate with each other continuously through vesicles
What is the function of the Golgi apparatus?
- Carbohydrate synthesis
- Post-translational modification or proteins and lipids - glycosylation (O-linked), phosphorylation and sulphation
- Sorting and dispatching station for products of the ER
structure of the golgi?
top to bottom:
- cis golgi network (CGN)
- cis cisterna
- medial cisterna
- trans cisterna
- trans golgi network )TGN)
where do early endosomes reside?
under plasma membrane
what do early endosomes mature into and how?
- they mature ino late endosomes
- by fusing with each other
- by fusion with a late endosome
where are late endosomes located?
near the nucleus
what do late endosomes do?
- sorting compartment in the endocytic pathway (endocytosis)
Describe the function of the lysosome as a recycling centre
Acid hydrolases - nucleases for nucleus degradation, proteases for protein degradation
pH very different from cytoplasm, most acidic environment of the cell (requires compartmentalisation to protect the cell)
ATP driven pump maintains pH via hydrogen ions
Digestion products diffuse out of the lysosome or need to be pumped out
Give an overview of vesicular transport
- Association of “cargo” with the area of the donor membrane will give rise to the vesicle
- Membrane distorted to form bud
- Detachment of bud to form vesicle
- Movement of vesicle containing cargo across cell to vicinity of recipient membrane
- Recognition of and binding to recipient membrane
- Fusion of vesicle with recipient membrane and release of cargo
Describe vesicular transport: budding and uncoating
- Cargo molecules bind to transmembranous receptors
- Curved coat proteins (COP I, COP II or clathrin) recruited and distort membrane
- Vesicle released by dynamin
- Vesicle rapidly uncoated
- Coat proteins have a curved structure to cause the bulging of the membrane
Describe the targeting aspect of vesicular transport
- Vesicles have surface markers which identify origin and cargo
- complementary receptors are displayed on the target membrane
- > 20 SNARE proteins work in pairs: each visicular v-SNARE has a complimentary target membrane t-SNARE
- Wrap around one another forming trans-SNARE complex: locks membranes together (docking) and mediates membrane fusion
Describe the targeting aspect of vesicular transport
- Vesicles have surface markers which identify origin and cargo
- complementary receptors are displayed on the target membrane
- > 20 SNARE proteins work in pairs: each visicular v-SNARE has a complimentary target membrane t-SNARE
- Wrap around one another forming trans-SNARE complex: locks membranes together (docking) and mediates membrane fusion
Describe the exit of cargo from vesicles at the Golgi apparatus in vesicular transport
COP II recruited to cause budding
Vesicles rapidly shed coat
Undergo homotypic fusion mediated by SNAREs
Resulting vesiclar tubular clusters moved along microtubules by dyneins to the Golgi
Fuse at Golgi and deliver contents
Cargo release mediated by pH decrease
Various proteins must be retrieved and have signal sequences (escaping ER proteins, receptor proteins)
Vesicles coated in COP I bud from VTC and Golgi are uncoated and transported back to ER (recycled)
Retrieval pathway can be subverted by bacterial toxins to gain entry to cytoplasm
Describe the constitutive secretory pathway of exocytosis
Trans-Golgi network (TGN) to plasma membrane
Proteins don’t require a signal to secreted through a pathway
Constant recycling of material (continuous communication)
Describe the regulated secretory pathway of exocytosis
Specialised secretory cells (hormones, neurotransmitters, digestive enzymes)
Prior to fusion, vesicle contents may be concentrated or processed
Describe the regulated secretory pathway of exocytosis
Specialised secretory cells (hormones, neurotransmitters, digestive enzymes)
Prior to fusion, vesicle contents may be concentrated or processed
explain how neurotransmitters are released by exocytosis
- docking
- priming
- signalling: ca2 intake-actio potential
- fusion: firing
How do tetanus and botulinum toxin interfere with synaptic signalling in exocytosis?
Tetanus and botulinum toxin are proteases which cleave trans-SNARE complexes
t-SNAREs needed for next batch of vesicles to dock are destroyed
Results in blocked synaptic transmission
Toxins are highly specific, only enter certain neurones
Outcome can be fatal
What is endocytosis?
- intake of molecules from the extracellular space and from the membrane
- nutrients
- recycling
what are the 3 ways endocytosis can take place?
- phagocytosis
- pinocytosis
- receptor-mediated endocytosis
explain pinocytosis
- recylcing of membrane and no spcecific uptake
- clathrin- coated pits
explain phagocytosis
- specialised white blood cell
Interaction receptor-phagocytosis trigger e.g. Ab-Epitope
Rearrangement of cytoskeleton (pseudopods formation)
Formation of phagosomes
Fusion of phagosome within lysosome (depends on a balance between +ve/-ve stimuli
What happens to endocytosed vesicles and their contents?
Fuse with early endosomes (sorting site for endocytosed molecules
Describe the process of recycling after endocytosis
Membrane and many receptors are sent to recycling endosome
Vesicles return to plasma membrane e.g. LDL receptor releases LDL in acidic early endosome and receptor returns to membrane for more cargo
Describe the process of transcytosis after endocytosis
vesicles return to different part of plasma membrane which transports material across the cell
E.g. maternal antibodies are carried across the gut epithelium by transcytosis
Describe the process of degradation after endocytosis
Cargoes and some receptors sent to late endosomes which mature into lysosomes
Macromolecules degraded and components used to make new molecules
E.g. LDL sent to late endosomes and cholesterol released by degradation of the rest of the particle in the lysosome
